Investigations on 10 MHz LGS and LGT crystal resonators

Materials in the LGT family are promising for designing bulk acoustic wave resonators with high quality factor. In our laboratory, we have manufactured a lot of planoconvex 10 MHz 5^th overtone Y-cut resonators using LGS (langasite La₃Ga₅SiO₁₄) and LGT (langatate La₃Ga_5.5Ta_0.5O₁₄) crystals. Our initial aim was to do noise measurements on these home-made resonators but problems occurred during manufacturing. It was the opportunity for further investigations. Indeed, we observed that the quality factor depends strongly on the energy trapping, the polishing method and the materials quality from one supplier to another. As for the quartz crystal, we have found that the material quality can be qualified by IR spectrometry whose resulting spectra exhibit absorption peaks more or less deep, linked to defects. These predominant criteria are not surprising but although they are nowadays quite well-defined in the case of quartz crystal resonators, they have to be defined again in the case of these LGS and LGT crystals. Then, a satisfying machining and polishing method has been first applied to elaborate high Q resonators. A comparison between different grades of LGS and LGT materials is established. In addition, LGT resonators are characterized by their motional parameters and frequency-temperature curves. Nevertheless, one of the main results is that the measured Q-f product is not the expected one. We present results of Q-factor versus radius of curvature and their comparison with the theoretical approach. It appears that an optimization should be performed. Right now the best resonator that we have made has got a Q-f product of 1.4 10¹³ on its 5^th overtone (1.7 10¹³ on its 9^th overtone). This result is slightly higher than the similar parameter obtained on a SC-cut quartz crystal resonator working at the same frequency.